door

n.

Middle English merger of Old English dor (neuter; plural doru) "large door, gate," and Old English duru (fem., plural dura) "door, gate, wicket;" both from Proto-Germanic *dur- (cf. Old Saxon duru, Old Norse dyrr, Danish dør, Old Frisian dure, Old High German turi, German Tür).

The base form is frequently in dual or plural, leading to speculation that houses of the original Indo-Europeans had doors with two swinging halves. Middle English had both dure and dor; form dore predominated by 16c., but was supplanted by door.

A door is what a dog is perpetually on the wrong side of. [Ogden Nash]